Cathode ray tube electrode structures



Nov. 7, 1939.

R R. LAW

CATHODE RAY TUBE ELECTRODE STRUCTURES Filed Feb. 10, 1958 ATTORNEY.

Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE CATHODE RAY TUBE ELECTRODE STRUC- TURES Application February 10, 1938, Serial No. 189,718

11 Claims. (Cl. 250158) My invention relates to cathode ray tubes and particularly to means for deflecting an electron beam over a target or fluorescent screen in two mutually perpendicular directions.

In cathode ray tubes for use in television transmission and reception and for Oscilloscopes, it is customary to deflect the electron beam in two mutually perpendicular directions to trace the desired pattern on the target which in television transmitting tubes is usually of the photosensitive mosaic type while in television receiving tubes and oscllloscopes the target is usually a fluorescent screen which becomes luminescent under the bombardment of the electrons in the beam. This two way deflection may be accomplished by electrostatic or electromagnetic fields or combinations of the two but for tracing high frequency phenomena on Oscilloscopes and for certain applications in television it is desirable to use electrostatic deflection because of the linear response obtainable over high ranges of frequency.

The conventional two way electrostatic deflection system, in which two mutually perpendicular deflection flelds are placed at different positions along the electron beam trajectory, has somedisadvantages: the beam length and consequently the length of the tube is greater than would be necessary if the two sets of deflection plates were in the same region; the width of the beam trace is increased, inasmuch, as the diameter of the electron beam at the screen is proportional to the distance between the electron source and the screen; and the inter action between the two electrostatic deflection flelds introduces fringing effects between the fields which tend to defocus the electron beam as it passes through and is deflected by these fields. Furthermore, in tubes having full electrostatic deflection systems the deflection sensitivity of the horizontal and vertical deflection system is un equal because the two systems are located at unequal distances from the target. To obtain elec- F trostatic deflection systems having equal deflection sensitivity it is necessary to use plates having diflerent shapes and lengths and in many cases requires the construction of special mounting jigs for use in assembly. If the two sets of deflection plates are in the same region the fringing effects are accentuated, resulting in defocusing of the electron beam and non-uniform deflection of the beam over the target. Although two way electrostatic deflection offers great flexibility over wide ranges of operating frequencies, these-disadvantages have decreased its usefulness to a considerable extent.

One object of my invention is to accomplish two way deflection with all the deflection plates located in the same region along the beam path, and without fringing and defocusing effects. A further object is to avoid inherent disadvantages of the conventional electrostatic deflection system for cathode ray tubes.

In accordance with my invention the electrostatic deflection system for cathode ray tubes is in the form of an open ended box having semiconducting walls of uniform resistivity joined at their edges, and positioned to permit the beam to pass through the box end to end. The deflection potentials are impressed on diagonally opposite corners of the box, the potential on each corner of the box at any instant being the same at all points along the corner. With corners of equal length and the deflection potentials thus applied symmetrically the radial deflection gradient within the box is homogeneous for all directions and magnitudes of the radial deflection gradient.

These and other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing in which:

Figure l is a diagrammatic view of a cathode ray tube incorporating my invention;

Figure 2 is a greatly enlarged perspective view of the deflection structure incorporated in the tube shown in Figure 1;

Figure 3 is a perspective view of one modification of the structure shown in Figure 2, and;

Figure 4 is a diagram indicating the operation of the deflection structure shown in Figure 2.

In the illustrative embodiment of my invention shown in Figure l, the tube comprises a highly evacuated glass envelope or bulb l with the tubular arm or neck section enclosing the conventional type electron gun and a frusto-conical section closed at the end remote from the gun structure and provided with a fluorescent screen or target 2 adapted to be rendered luminescent under the impact of the electron beam.

The electron gun assembly in the neck section of the bulb I is of the conventional type and comprises a cathode 3 from which an electron stream may be drawn, a control electrode 4 connected to the usual biasing battery, and the first anode 5 maintained positive with respect to the cathode 3. The electrons emitted from the oathode 3 are directed through the first anode 5 and concentrated into an electron scanning beam focused on the surface of the target 2 by a second anode 6 which is preferably a conductive coating on the inner surface of the neck section extending between the first anode B and the fluorescent screen 2. The first anode 5 and the second anode 6 are maintained at the desired positive potentials with respect to the cadahode 5 by a battery I.

In accordance with my invention the electron beam is deflected from its normal path as it leaves the first anode of. the electron gun by electrostatic deflection structure of box-like form made of semi-conducting material and located between the end of the first anode I and the target 2, with means for maintaining each corner of the box at substantially the same potential throughout its length. As bestshown in Figure 2, the electrostatic deflection structure preferably comprises four rectangular plates I051 I, I2 and I3 assembled to form an open ended box having a cross-section which is rectangular, meaning any quadrilateral figure having four right angles and including a square. The four plates consist of semi-conducting material and are of the same resistivity throughout. The box is constructed so that a potential impressed on a corner is the same at all points along the comer preferably by extending along the length of the corner a conductor embedded in or joined to the plates at the corner. A convenient way of making the box is to bevel the plates at their longitudinal edges and join them at the longitudinal bevelled edges with an electrically conducting cement. It is usually desirable to make the deflection box with sides of equal width and therefore of square cross-section normal to the undeflected path of the electron beam through the box to obtain equal deflection sensitivity in both directions of deflection, although for certain applications the adjoining sides may be made of unequal width to obtain unequal deflection sensitivities; for example the width of the plates l0 and I! may be less than or greater'than the width of plates Ii and i3.

In the electrostatic deflection structure shown in Figure 2, the plates I 0-l 3 are of semi-conducting material, or of a porous material which may be impregnated with electrically conducting material to render themsemi-conductive, and of a width and length which gives the desired defiection sensitivity. As a plate material I have found particularly advantageous a ceramic body of high porosity such as a body of alumina particles sintered together with a suitable bonding agent. Sintered alumina having a porosity of about 44%. is particularly suitable and is known commercially as Norton RA 225 Alundum. The material is cut to the desired size, bevelled along two opposite edges, and cemented together along the bevelled edges with electrically conducting cement I4 such as burnished silver paste, or other conducting material, to form the box-like structure shown in Figure 2. Silver paste suitable for this purpose may comprise fifteen parts of finely powdered silver, 10 parts of Burgundy pitch, and one part of bismuth subnitrate mixed with sufficient solvent such as oil of rosemary to yield a workable paste cement. The contiguous edges of the plates forming the box-like structure are pref erably bevelled at an angle of approximately 45 with the plane of the plate, and cemented together with the electrically conducting cement H. The cement l4 extends the length of the corners and provides means for impressing a single potential at all points along each corner and is of loidal graphite in boiling water, and allowed to soak up the suspension for a period of about one minute.

It will be apparent from the showing of Fig. 2 that the electrical conductors along each corner of the assembly have an area exposed to the inner surface of the deflection assembly which is negligible in comparison with the area of the plates lil'l3. This small exposed area of the conductors is desirable to prevent distortion of the desired deflection field adjacent the corners of the box.

The impregnating material may be deflocculated graphite, such as the material commercially known as "Aquadag, which is particularly good for this purpose. When this particular material is used I make a mixture of one part of Aquadag to one hundred parts of water, and use it with the grade of Alundum indicated above to produce a ceramic body having an electrical resistance of approximately 4,000 ohms/cm The proportion of Aquadag to water is not critical, and the specific resistance of the material comprising the plates l0-l3 may be varied over a wide range by varying the concentration of the Aquadag suspension. It is very desirable that the electrical resistivity of the plates be made as uniform as possible, so that if the resistivity obtained by following the above treatment is less than desired, I increase it to the desired value by dipping the electrode in water several times, each dipping followed by drying, rather than by decreasing the concentration of the Aquadag" in the suspension.

Figure 3 shows a deflection structure made in accordance with my invention which is particularly adapted to compensate keystone distortion effects which are introduced when the plane of the target 2 is not perpendicular to the longitudinal axis of the tube. Thus if the target is at .an acute angle such as an angle of 60 with the longitudinal axis of the tube, represented in the figure as the center line IS, the outline of the area scanned by the cathode ray beam has a keystone shape rather than the desired rectangular shape. This keystone distortion may be compensated for by so forming the deflection structure that the cathode ray beam is under the influence of the electrostatic deflection fields for a greater period of time when the beam is directed toward that portion of the target which is nearer the electron gun.

As shown in Figure 3 the tubular deflection structure is formed as an obliquely truncated rectangular prism, with two diagonally opposite corners of equal length, and the other two corners of unequal length. The plates Hi -l3 are each shaped so that two edges are parallel, the third edge perpendicular to the two parallel edges and the fourth edge at an angle other than 90 with the parallel edges. The term right trapezoid" will be used to define the shape of the plates when formed in this manner. The structure is mounted in a tube having an inclined target preferably with the apex of the truncated prism toward the target.

The electrical connections for applying the deflection potentials to diagonally opposite corners of the box may conveniently be made by drilling small holes in the cement atthe corners, filling the holes with additional cement, inserting the wires or leads I5 and I8 which are used as electrical conductors and supports for the assembly, and firing the assembly with the inserted leads in a vacuum furnace operated at 500 C. for approximately 30 minutes to set the cement.

This firing step may be avoided however if the leads are attached at the time the plates are cemented together to form the box.

Many modifications of this arrangement for coincidental horizontal and verticalelectrostatic deflection will occur to one'skilled in the art.

. any suitable conducting material.

For example, the deflection box may be made from some other porous non-conducting material than Alundum and may be made semi-conducting by impregnating it with a desired amount of In addition, the impregnating material may be a solution containing a metal salt following the application of which the assembly may be fired in vacuum to reduce the metal salt to metal thereby rendering the non-conductor partially conducting. Instead of making the plates III-43 of porous material they may be made of non-porous material which is coated on the inner surface with a conducting material having the desired electrical resistance. In addition, it should be pointed out that the relative dimensions of the deflection box may be varied through wide limits to satisfy the requirements of any particular application. Deflection structure using plates two inches long and one inch wide has given a deflection sensitivity of I connected to the electrical conductor along the longitudinal junction of plates II], II and I 2, I3 respectively while the horizontal deflection potentials are applied through the current carrying leads I6 and I8 to the conductor along the longitudinal junction between plates II, I2 and I0, I3.

Figure 4 is a diagram indicating various conditions of operation, the central circle representing the target 2, with an inscribed rectangle 20 in dashed lines representing the boundaries of the pattern traced on the target by the electron beam. Within the rectangle 20 there is shown a square 2I representing diagrammatically the projection of the deflection structure on the target 2 and within the square 2| a rectangle 22 in dashed lines showing the area swept out by the beam as it leaves the deflection structure. The area swept out by the beam as it leaves the deflection structure, represented by the rectangle 22, is a miniature of the area swept out by the beam on the target and represented by the rectangle 20. The eight dots on the rectangle 22 correspond in position to those on the rectangle 20 and represent eight positions assumed by the electron beam both as it leaves the deflection structure and as it impinges on the target. Eight conditions of operation are shown in Sub Figures A to H inelusive, the squares surrounding the circle or target 2 representing eight views of the cross-section of the structure shown in Figure 2, the horizontal and vertical deflection potentials shown as being impressed upon the diagonally opposite corners. The dashed parallel lines in Sub Figures A to F represent parallel planes of equipotential regions which extend from end to end of the deflection box and are shown merely as an aid to describe the operation under various conditions.

Sub Figure A indicates the operation of the deflection system with horizontal and vertical deflection potentials applied to the box for projecting the electron beam toward the upper left hand corner of the fluorescent screen or target 2. Horizontal deflection potentials are applied to the system such as 40 volts positive to the control lead I6, and 40 volts negative to the lead I8, to

produce horizontal deflection to the left hand edge of the fluorescent screen. At the same time vertical deflection potentials of 30 volts positive and 30 volts negative are applied to the leads I5 and H which deflect the beam vertically resulting in the beam impinging on the screen in the upper left hand corner. Sub Figure B indicates the position at which the beam impinges on the target with similar horizontal deflection potentials but with no vertical deflection potentials. This condition of deflection potentials produces impingement of the beam at the center left hand edge of the target 2 while under the same horizontal deflection conditions but with the vertical deflection potentials shown in A reversed, the beam impinges on the screen at the lower left hand corner as indicated in Sub Figure C. Sub Figures D, E and F indicate the operation of the deflection structure with vertical and horizontal deflection potentials as in A, B and C but with the polarity of the horizontal deflection potentials reversed. 'These conditions produce beam deflection to the upper right, center right and lower right portions of the target 2. With no horizontal potentials applied to the leads I6 and structure operates because of potential differences or gradients set up along the inner surfaces of the electrodes or deflection plates when current is made to flow in the vertical and horizontal deflection circuits. It is therefore believed that there are formed between the plates, regions of equal potential which regions may be represented as shown in Figure 4 by dashed lines as extending through the space enclosed within the box-like structure, these lines representing parallel planes of equi-potential regions extending longitudinally of the deflection box. Thus, if the semi-conducting walls of the box-like structure have uniform resistivity throughout and the deflection potentials are applied symmetrically, the electrostatic equi-potential lines are straight from one wall of the box to the opposite wall so that the field for all directions and magnitudes of the radial deflection gradient will be homogeneous. With this deflection box arrangement, the two mutually perpendicular deflection flelds are applied co-incidentally and produce a uniform radial deflection gradient with no defocusing effects due to interaction between the two flelds and since the overall length of the deflection system is less than one-halt that of the conventional system where the plates are eflective over different portions of the path of the electron beam, the electron gun may be moved up closer to the screen to give a smaller and more intense beam trace.

While I have indicated the preferred embodiments of my invention of which I am now aware and have indicated'the specific application as directed to a cathode ray tube having a fluorescent tar'get, it will be apparent that my invention is by no means limited to the exact forms illustrated or to the use of my. invention in cathode ray tubes incorporating fluorescent targets but that many variations may be made in the particular structure used and the purpose for which it is employed such as by replacing the fluorescent target with target electrodes of the mosaic type or light valve t'ype without departing irom the scope of my invention as set forth in the appended claims.

What I claim as new is:

l. A tubular cathode ray electrostatic deflection structure comprising four nonmetallic rectangular plates of electrically semi-conductive material joined at their contiguous edges to form an open ended box enclosing an area of square cross-section, and an electrical conductor having a negligible resistance in comparison with that of the semi-conductive material extending along each corner of the box and electrically connected throughout its length to the adjoining plates to produce when each corner is energized a potential gradient in the semi-conductive material between adjacent edges of the box.

'2. An electrostatic deflection structure for a cathode: ray tube comprising iour rectangular nonmetallic plates of electrically semi-conducting materialhaving' aresistance of at least 4000 ohms/cm}, and metal conductors interposed between and joined to the plates at the corners to form an open ended box enclosing an area of rectangular cross-section.

3. An electrostatic deflection structure for a cathode ray tube comprising an open ended box including four nonmetallic plates of semi-conducting material and of rectangular'cross-section and means including an electrical conductor of low electrical resistance in comparison with that of the semi-conducting material electrically connected to the box along each corner for maintaining each corner 0! the box at substantially the same potential throughout its length.

4. A tubular electrostatic structure for a cathode ray tube comprising four rectangular nonmetallic plates of electrically semi-conducting material having a resistance of approximately 4000 ohms/cmP joined at their contiguous edges to form a box enclosing an area of rectangular cross-section, and means extending along each corner of the box to impress a single potential at all points along each corner. A

5. A tubular electrostatic deflection structure for a cathode ray tube comprising four rectangular plates oi material having a resistance of approximately 4000 ohms/cm? forming an enclosure having a rectangular cross-section and each having two opposite edges beveled, said plates having their contiguous edges cemented together along the beveled edges with electrically conductive material of negligible resistance with respect to that of the plates extending the length of said edges.

6. A tubular electrostatic deflection structure for a cathode ray tube comprising tour rectangular plates having an electrical resistance of at least 4000 ohms/cm each having two opposite edges beveled at an angle of approximately 45 to the plane of the plate, electrical conductors in electrical contact with and extending along the beveled edges to form an enclosure having a rectangular cross-section, and means to apply an electric potential to each of said conductors.

7. A tubular electrostatic deflection structure 'ior a cathode ray tube comprising tour electrically semi-conductive plates having a resistance of at least 4000 ohms/cm? joined along their contiguous edges to form a box having the shape of an obliquely truncated rectangular prism and electrical conductors of negligible electrical resistance extending along and in electrical contact with the said contiguous edges, the surface 01 said conductors exposed to the interior of said box being negligible in comparison with the area of said plates.

8. A cathode ray deflection structure comprising four electrically semi-conductive plates having a resistance of at least'4000 ohms/cm. each having the shape of a right trapezoid joined along their parallel edges with electrically conductive material of negligible resistance in comparison with that of the plates to form a tubular obliquely truncated rectangular prism.

9. A tubular electrostatic deflection structure for a cathode ray tube comprising an open ended box of electrically semi-conductive material having a resistance of at least 4000 ohms/cm. having the shape of an obliquely truncated rectangular I prismwith two of the diagonal corners of equal 'joined at their edges to form an open ended electrode assembly of rectangular cross-section,

an electrical conductor of lower electrical resistivity than the semi-conducting material along each of the corners and electrically connected thereto throughout the length of each corner, and means to apply an electric potential to each of said conductors.

11. A cathode ray tube including in an evacuated envelope an electron gun, a target and a tubular electrostatic deflection structure between said electron gun and said target and surrounding a portion of the path between said gun and said target, said structure comprising an open ended box havingtour nonmetallic plates of semiconducting material having an electrical resistance of at least 4000 ohms/cm; and of rectangular cross-section and means extending alongand adjacent each corner of the box to impress a de- RUSSELL R. LAW. 

